10 - Airside Design.indd - Oregon State Library: State Employee
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1. Code Language continued from page 1 3 At least 75 percent of the energy for reheating or for providing warm air in mixing systems comes from a site recovered or site solar energy source 4 Zones where specified humidity levels are required to satisfy process needs such as com puter rooms museums and areas of hospitals 5 Zones with a peak supply air quantity of 300 cfm 142 L s or less 1318 2 6 Zone isolation controls A system serving multiple occupancies or floors in the same building shall be independently zoned and equipped with isola tion devices capable of automatically shutting off the supply of conditioned air and outside air to and from each isolated area Each isolated area shall be con trolled independently and satisfy temperature setback Section 1317 4 2 and optimum start control require ments The central fan system air volume shall be re duced through fan speed reduction Exceptions A cooling system less than 240 000 Btu hr 70 kW or a heating system with less than 300 000 Btu hr 88 kW total capacity 1318 2 7 Separate air distribution systems Zones with special process temperature requirements and or humidity requirements shall be served by separate air distribution systems from those serving zones requir ing only comfort conditions or shall include supple mentary control provisions so that the primary sys tems may be specifically controlled for comfort purposes only Exceptions Zones2. requiring only comfort heating or comfort cooling that are served by a system primarily used for process temperature and humidity control provided that 1 The total supply air to those comfort zones is no more than 25 percent of the total system supply air or 2 The total conditioned floor area of the zones is less than 1 000 square feet 90 m Continued from page 1 flow setpoint on VAV boxes The code establishes an upper limit and the intent is to reduce airflow as much as possible during low cooling load periods before using reheat There are three options for calculating this limit and the designer can apply the method that results in the highest airflow There is also an exception for zones with peak airflow no greater than 300 cfm The first calculation option 30 percent of peak air flow and the third method 0 4 cfm per square foot are fairly simple The second calculation method is more complicated than the other two because it is based on overall system outdoor air ventilation rate calculations The second method typically applies to zones with high occupant density and it allows system designers to optimally solve Equa tion 6 1 in ASHRAE Standard 62 1 2004 That equation shows that the amount of outdoor air required for a system is a function of how much air is supplied to the critical zone in the system The higher the supply air rate to the critical zone the less outdoor air is required at the air handler T
3. FE NORTHWEST ENERGY EFFICIENCY ALLIANCE 3 www nwalliance org Photo on page 1 c o Warren Gretz DOE NREL 12 05 ODOE CF 125 Fact Sheet 10 Non residential code HVAC fact sheets include Ventilation Controls System Economizers Exhaust Air Heat Recovery Airside Design Requirements e Hydronic Design Controls Airside Controls Large Volume Fan Systems Air Transport Energy Simple vs Complex HVAC Systems Continued from page 3 clean rooms or other special manufacturing spaces computer rooms or other special electronic equipment The energy required to maintain conditions in these process spaces is typically greater than for spaces where human comfort is the only criterion The intent of section 1318 2 7 is that these spaces with process requirements be served by separate systems so that energy is not wasted by overconditioning the other portions of the building In some cases it may not be practical to provide separate systems and the code allows some exceptions These exceptions permit a small amount of comfort conditioned space to be served by systems that primarily serve the process areas The comfort areas must either receive no more than 25 percent of the air flow or must be less than 1 000 sq ft For More Information See also the fact sheets covering Airside Control Requirements Air Transport Energy Economizers Ventilation Controls for High Occupancy Areas and Exhaust Air Heat R
4. Oregon Non Residential z Building Energy Code ne a ve Airside System Design Requirements These sections fall under 1318 2 Complex Control Systems They are grouped here as the topic Airside Design because they affect the fundamental system design including system type selection and system layout Simultaneous Heating and Cooling The intent of section 1318 2 1 is to minimize energy waste by limit ing the amount of simultaneous heating and cooling that occurs in HVAC systems Simultaneous heating or cooling occurs when sup ply air that has been previously cooled is reheated supply air that has been previously heated is recooled or supply air that has been cooled is mixed with supply air that has been heated The most important consequence of this requirement is that certain HVAC system types are prohibited except under special circumstances The following system types are generally not allowed e Constant volume reheat system A single air handler that pro vides a constant flow of cool air to multiple zones with a hot water or electric resistance coil at each zone to reheat the air and control space temperature e Two deck multizone system A single air handler that serves multiple zones and delivers a mixture of heated and cooled air to each zone in a separate duct The ratio of heated and cooled air is varied separately for each zone to control space tempera tures e Constant volume dual duct system Similar in princ
5. ce with separate air distribution requirements must be recorded on Form 4b line 10 1 Variable air volume VAV systems which during periods of occupancy are designed to reduce the air supply to each zone to a mini mum before reheating recooling or mixing takes place This minimum volume shall be no greater than the larger of the following 1 1 Thirty percent of the peak supply volume 1 2 The minimum required to meet ventila tion requirements unless increasing the volume to critical zones zones with the highest ratio of outside air to total supply air beyond the minimum ventilation re quirements results in a decrease in overall outside air required by the HVAC system An increase beyond minimum ventilation rates shall not be applied to more than 20 percent of the zones with reheat on any one system or V3 0 4 cfm ft 2 L s per m of zone condi tioned floor area 2 Zones where special pressurization relation ships or cross contamination requirements are such that variable air volume systems are impractical such as some areas of hospitals and laboratories Systems which use this exception and supply heated or cooled air to multiple zones shall include controls which automatically reset supply air temperatures by representative building loads or by outside air temperature unless it can be shown that supply air temperature reset increases overall building annual energy costs Code Language continues on page 2
6. ecovery See the Advanced VAV System Design Guide for detailed recom mendations on VAV box minimum airflow settings Available at http www newbuildings org mechanical htm See also ASHRAE Standard 62 1 2004 Ventilation for Acceptable Indoor Air Quality and the Standard 62 1 2004 User s Manual for guidance in determining ventilation rates and in performing criti cal zone calculations Available at www ashrae org Examples An office building includes a 200 square foot retail space adjacent to the lobby as well as a 2 000 square foot cafeteria May these spaces be served by the central HVAC system Yes these zones can be served by the central system The separate air distribution system requirements 1318 2 7 do not apply in this case because the primary system serving the office spaces is being controlled only for comfort purposes However the zone isolation requirements of Section 1318 2 6 require that the system be able to shut off air flow to and from these different occupancies separately from the office occupancies In addition if this is a multi story building then each floor must have isolation controls capability Airside System Design Requirements Oregon Non Residential Building Energy Code
7. elocity of the buoyant warm supply air will be too low to provide adequate mixing with room air Thermal stratification warm air at the ceiling cool air at the floor is more likely to occur as the supply air temperature gets warmer more buoyant and the velocity of air leaving the diffuser decreases There are some strategies to combat stratification in heating mode Parallel fan powered VAV boxes have fans that turn on to increase air flow in heating mode and draw the extra air from the return air plenum In addition to improving comfort in heat ing mode parallel fan powered boxes will reduce or eliminate the need for reheat Series fan powered VAV boxes are another Continues on page 3 Continued from page 2 though less desirable option which maintain constant discharge air flow while allowing the primary ventilation airflow to de crease to a minimum setpoint A series fan powered box provides good comfort performance but is generally less energy efficient because it induces warm plenum air at all but peak cooling times leading to increased cooling requirements as well as higher fan energy Another alternative is to use separate perimeter heating systems such as radiant panels or baseboard heaters instead of reheat coils in VAV boxes In this strategy the VAV box air flow can be allowed to drop to a low minimum without concern of stratification occuring and the separate heating system prevents overcooling Zone Isolation Contr
8. ensity cases such as a meeting room or classroom If that is the case then the ventilation rate can be used as the upper limit Finally there is an exception in Section 1318 2 1 that allows the minimum flow to be set even higher if doing so will reduce the overall outdoor air ventilation rate for the central air handler Is a triple deck multizone system which has hot deck cold deck and neutral no heating or cooling deck allowed under the simultaneous heating and cooling requirements of Section 1318 2 1 Yes as long as the controls do not allow the mixing of air from the cold and hot decks In other words the controls must ensure that the air delivered to each zone is a mix of cold and neutral or hot and neutral Q The VAV system in a multi story office building is being provided with motorized dampers on the supply and return ducts at each floor to provide zone isolation control The toilet rooms are exhausted by central fan Is it also necessary to provide isolation dampers for the toilet exhaust at each floor Yes Section 1318 2 6 requires that the air flow to and from each isolation zone in this case each floor is capable of being shut off Therefore motorized dampers in the exhaust duct will be required at the connection to each floor Oregon Non Residential Building Energy Code Airside System Design Requirements Examples A two story building is served by a water loop heat pump system consisting
9. he designer would determine which is more energy efficient increasing outdoor air intake and minimizing reheat at the critical zone or increasing the supply air rate and reheat energy required at the critical zone and mini mizing the outdoor air rate This upsizing of the minimum flow may only be applied to a maximum of 20 percent of the zones with reheat on each system The designer should submit calculations demonstrating that increasing the volume to critical zones reduces overall outdoor air fraction A detailed discussion of ventilation rate calculations is beyond the scope of this fact sheet and more details are available in ASHRAE Standard 62 1 2004 and its User s Manual When specifying VAV box settings the designer should consider the energy savings opportunities for minimum airflow set points that are significantly lower than upper limits set by the energy code Reducing the minimum flow set point saves fan energy as well as cooling and heating energy Studies of real buildings have shown that many zones operate at their minimum flow for a majority of the time because cooling loads are often lower than predicted by the designer A good resource for recommendations is the Advanced VAV System Design Guide There are several potential design strategies that provide good comfort and ventilation performance in heating mode while al lowing low minimum settings in VAV boxes The potential prob lem with low air flow is that the v
10. iple to the multizone system except that the mixing occurs at the zone level instead of within the air handler Those system types are allowed in only three special cases 1 where constant airflow is necessary to maintain pressure rela tionships between spaces in order to prevent cross contamination 2 where reheat is necessary to maintain special humidity condi tions and 3 where reheat is provided by recovered waste heat or solar heat This code section generally requires the use of one of the following system types e Variable air volume VAV with or without reheat e VAV dual fan dual duct e Single zone systems such as fan coils packaged rooftop units or water source heat pumps For VAV systems section 1318 2 1 regulates the minimum air Continues on page 2 Code Language 1318 2 1 Simultaneous heating and cooling Zone thermostatic and humidistatic controls shall be capable of oper ating in sequence the supply of heating and cooling energy to the zone Such controls shall prevent reheating recooling and mixing or simultaneous supply of air that has been pre viously mechanically heated with air that has been previously me chanically cooled Exceptions OREGON DEPARTMENT OF ENERGY Documentation The simultaneous heating and cool ing requirements are documented on Form Ab Line 4 as well as Worksheet 4k Zone isolation controls are documented on Form 4b Line 11 Complian
11. ntrol DDC system should be able to provide this level of control However the code also requires that return air flow from the isolation zone be shut off and additional motorized dampers in the return air path will still be necessary The code also requires that central systems with zone isolation controls use fan speed control to throttle back flow when one or more zones is shut off Most large central systems will already meet this requirement because variable speed fan control is now common for duct pressure control in new VAV systems Separate Air Distribution Systems Many buildings include spaces that require special environmental conditions Examples of such spaces include hospital operating rooms laboratories special material storage such as rare books Continues on page 4 Examples A 1 000 square foot space is served by a VAV box sized for 1 500 cfm peak airflow What is the upper limit on the minimum air flow setting for the VAV box Section 1318 2 1 sets the limit at the largest of 30 percent of peak airflow 0 4 cfm per square foot or the amount required for ventilation In this case the 30 percent option yields 450 cfm 30 percent of 1 500 cfm and the 0 4 cfm per square foot option yields 400 cfm Using the larger of these two results allows the minimum VAV box flow to be set as high as 450 cfm However it is possible that the outdoor air ventilation rate required for this space would exceed 450 cfm for high d
12. of 25 heat pumps that total 100 tons of cooling capacity A central air handler delivers ventilation air to each heat pump This air handler has a heating coil to temper outdoor air during the winter How do the zone isolation controls apply If the heating capacity of the central air handler is 300 000 Btu hr or more then isolation capability is required for delivery of outside air to each floor and exhaust or return air paths from each floor would require isolation dampers A data center consists of computer rooms with special temperature and humidity requirements as well as supporting office and storage spaces Can the support areas be served by the same air handler that serves the computer rooms Section 1318 2 7 of the code allows the support areas to share the computer room air handler under two conditions 1 the total floor area of the support rooms is less than 1 000 sq ft or 2 the air flow provided to the support areas is less than 25 percent of the total system air flow Find OutMore Copies of code Oregon Building Officials Association phone 503 873 1157 fax 503 373 9389 Technical Support Oregon Department of Energy 625 Marion Street NE phone 503 378 4040 Salem OR 97301 3737 toll free 800 221 8035 www oregon gov energy fax 503 373 7806 This fact sheet was developed with funding from the Northwest Energy Efficiency Alliance and the US Department of Energy under contract DE FG51 02R021378
13. ols The intent of Section 1318 2 6 is to avoid conditioning the whole building at times when only portions are occupied These zone isolation control requirements apply to systems that serve more than one occupancy or multiple floors At a minimum such sys tems must be able to serve each occupancy or each floor inde pendently while shutting off air flow to and from the other areas served by the system In addition each isolation area must have independent time of day setback and optimal start controls These requirements do not apply to small HVAC systems Cooling systems smaller than 20 tons 240 000 Btu hr of cooling capac ity are exempt Heating systems with capacity of less than 300 000 Btu hr are also exempt A straight forward option for complying with these requirements is to include motorized dampers that shut off supply and return ducts to each isolation zone In a multi story building this option might consist of dampers located at the connections to the supply and return air shafts to isolate each floor The local fire official may allow the use of fire smoke dampers to serve this purpose as long as they are wired so that life safety controls take precedence over off hour controls A VAV system might comply if it has the capability to set the oper ating schedule of each zone or at least sets of zones independent ly so that the VAV box damper may be shut on some zones while others continue to operate A direct digital co
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